DC motor systems have ordinarily been used where high performance, low cost electric drives are required, as in the propulsion systems of electric vehicles. Although DC motors themselves are relatively complex, high performance controllers for them are simpler and less expensive than those for AC motors. AC motors themselves are generally of simpler structure and have the advantages of lower cost, compact size, low weight, and high efficiency, but their controllers are complex and expensive.
The cost of the inverters that are required for providing controlled power to AC motors from storage battery sources is expected to decrease as inverter systems come into greater use. For the future, therefore, the principal problem impeding the adoption of AC motors for such applications as electric vehicle drive systems is the relatively poor operating performance of the AC controller and inverter apparatus that are ordinarily used for supplying power to the AC motors.
Current mode control for operating polyphase AC motors may result in logic simplification. The current flowing into the motor terminals has been measured, and compared with current reference signals that were generated by a controller. The resulting error signals have been employed to control the switching of a polyphase inverter by gating the main semiconductors (often transistors) of the inverter on and off. Pulse-width-modulated burst of voltage from the DC power source are in that way rapidly switched about to appropriate phase terminals of the AC motor. A squirrel cage induction type of AC motor is favored for such applications because of its reliability, simplicity, and low cost.
A further important improvement in the control of AC motors is "field-oriented control", which will be discussed below. Field-oriented control is described in published technical literature, for example in the following publication:
Kaufman, George: Garces, Luis; and Gallagher, Gerard, "High Performance Servo Drives For Machine-Tool Applications Using AC Motors". Institute of Electrical and Electronic Engineers, IAS Annual Meeting Conference Record, PP 604-609, 1982.
Current mode controllers have been of two principal types: One was the hysteresis regulator: the other was a naturally sampled triangular wave comparison regulator with proportional plus integral error control. Numerous variations of these types have also been employed with greater or less success. Descriptions of these two prior art control systems have been deferred herein to the end of the Detailed Description of the present invention, in order that important differences between the present invention and the two main prior art systems could be more clearly described.